1. Field of the Invention
This invention relates generally to a fuel cell system including a fuel cell stack and an electrical energy storage device and, more particularly, to a fuel cell system including a fuel cell stack and an electrical energy storage device where the electrical energy storage device is separated into more than one storage bank so that the fuel cell stack has a high enough voltage to charge the electrical energy storage device.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack.
Most fuel cell vehicles are hybrid vehicles that employ a supplemental power source or electrical energy storage device (EESD) in addition to the fuel cell stack, such as a high voltage DC battery or a super-capacitor. The EESD provides supplemental power for the various vehicle auxiliary loads, for system start-up and during high power demands when the fuel cell stack is unable to provide the desired power. The fuel cell stack provides power to an electric traction motor through a DC high voltage bus line for vehicle operation. The EESD provides supplemental power to the voltage bus line during those times when additional power is needed beyond what the stack can provide, such as during heavy acceleration. For example, the fuel cell stack may provide 70 kW of power. However, vehicle acceleration may require 100 kW of power. The fuel cell stack is used to recharge EESD at those times when the fuel cell stack is able to provide the system power demand. The generator power available from the traction motor during regenerative braking is also used to recharge the EESD.
In the hybrid vehicle discussed above, a bidirectional DC/DC converter is sometimes employed to step down the DC voltage from the EESD to match the EESD voltage to the bus line voltage dictated by the voltage output of the fuel cell stack and step up the stack voltage during EESD recharging. However, DC/DC converters are relatively large, costly, heavy and unreliable, providing obvious disadvantages. It is desirable to eliminate the DC/DC converter from a fuel cell vehicle including a supplemental power source.
There have been various attempts in the industry to eliminate the DC/DC converter in fuel cell powered vehicles by providing a power source that is able to handle the large voltage swing from the fuel cell stack over the operating conditions of the vehicle. Certain types of batteries have also been used to eliminate the DC/DC converter in vehicle fuel cell systems. However, these systems are typically limited by the ability to discharge the battery beyond a certain level. In other words, these types of batteries could be damaged as a result of large voltage swings on the DC bus line during the operation of the system.
In one particular fuel cell system hybrid design, the EESD has a higher voltage potential than the idle voltage of the fuel cell stack, which is the stack maximum, so as to allow the EESD to recapture regenerative braking power from the electric traction motor. It may be desirable to fully charge the EESD at system shut-down so that the EESD can be a capacitor device that would be subject to dissipation over long periods of system shut-down. Thus, the higher the voltage of the EESD at system shut-down the more likely the EESD will have enough charge to operate system loads, such as the cathode air compressor, at the next vehicle start-up. It is possible to use a low voltage battery, such as a 12-volt battery, and a 12 volt to high voltage boost converter to provide energy in addition to the fuel stack power to fully charge the EESD. However, such a recharging scheme offers a number of challenges in design considerations, including the size of the 12 volt battery.